A field effect transistor (FET) can be used as an efficient pass through switching device that can selectively couple a signal from a signal source to a sink for that signal. Typically, pass through FETs are selected to have a low ON resistance (Ron) and a high OFF resistance (Roff). The low Ron enables a signal to pass through the FET with little loss or other affects caused by the FET.
For a FET, Ron is a function of the gate to source voltage, Vgs. As Vgs increases, the channel size for a FET also increases which reduces Ron for the FET. Accordingly, Ron has an inverse relationship to Vgs. Of course, to operate effectively as a pass through FET, Vgs can only be increased up to the gate oxide breakdown voltage or other maximum recommended operating voltage for the FET. This places an upper limit on how high Vgs can be set. Since a low Ron is typically desired, a FET used as a pass through switching device should have Vgs set to a point slightly below its gate oxide breakdown voltage.
Vgs, as the gate to source voltage, is dependent upon both the gate voltage and the source voltage; notably, the difference between the gate voltage and the source voltage. In a FET used as a pass through switching device, the gate voltage is controlled, while the source voltage is uncontrolled. The gate voltage is controlled to set the FET either ON (e.g., in a conducting state) or OFF (e.g., in a non-conducting state). The source voltage, however, is based on the voltage from the signal source; and is therefore uncontrolled. For example, when the FET is ON the source voltage is substantially equal to the voltage of a signal propagating into the source of the FET (referred to herein as the “input signal”). Accordingly, with the gate voltage set at a constant value while the FET is ON, Vgs will vary according to changes in the input signal.
This variation in Vgs causes a corresponding variation in Ron for the FET. In order to reduce the variation in Vgs, circuits have been designed such that the gate voltage of the FET tracks voltage variations in the input signal. These circuits use a non-switching amplifier (e.g., class A, B, AB amplifier) to generate the adjustable gate voltage for the FET. These non-switching amplifiers are typically configured such that the gate voltage continuously tracks the input signal (e.g., in an analog manner). Accordingly, these circuits typically account for all voltage swings of the input signal, including the voltage swings within a “constant” AC input signal (e.g., an AC signal having a constant common mode). That is, in these circuits a “constant” AC input signal will cause variation in the gate voltage such that Vgs remains substantially constant as the AC input signal swings from high to low.